Miyazaki, K. (National Institute of Advanced Industrial Science and Technology) | Tenma, N. (National Institute of Advanced Industrial Science and Technology) | Endo, Y. (Chuo Kaihatsu Corporation) | Yamaguchi, T. (Toho University)
Geomechanical behaviors of methane-hydrate-bearing sub-seabed layers have not been sufficiently clarified, although they are essential to ensure sustainable exploration of methane hydrate in marine sediments. In particular, the viscoelastic properties of methane-hydrate-bearing layers are thought to have great significance in the long-term prediction of the geomechanical behaviors. It has been clarified that methane-hydrate-bearing sand has significantly strong time dependence for a geomaterial. Thus a constitutive equation used for the long-term prediction of the geomechanical behaviors is required to consider the viscoelasticity (time dependence) of methane-hydrate-bearing sand. To date, however, there has not been any viscoelastic constitutive equation modeled based on experimental results. This study aims to discuss the viscoelastic properties of methane-hydrate-bearing sand specimens obtained from drained triaxial compression tests and propose a viscoelastic constitutive equation. The results of three types of triaxial compression tests, e.g., constant-strain-rate test, constant-stress-rate test and creep test, on artificial methane-hydrate-bearing sand specimens are reviewed, mainly focusing on the viscoelastic behaviors. Then, a nonlinear viscoelastic constitutive equation is proposed for methane-hydrate-bearing sand, considering the strain-rate and stress-rate dependences of mechanical properties and creep behaviors. The model prediction fits well with the test results of stress-strain relationships in constant-strain-rate and constant-stress-rate tests before the peak failure point and primary creep behavior.
Percussive drilling with a rock drill is a prevailing modern technique for blasthole drilling and rock bolting. In a rock drill, force and energy for rock fragmentation are generated by repeated collisions between a piston and a shank rod, and then transmitted to a drill bit through extension rods in the form of stress waves. Due to the reason that stress waves play a crucial role in conversion of the kinetic energy of impact into drilling work, understanding stress wave propagation in rods is important in optimizing the design of rock drills. Lundberg deeply studied the stress wave propagation by experiments and built its numerical models based on the one- and three-dimensional theories of elastic waves. Okubo et al. proposed two kinds of sleeve-type joint models according to the result of percussive drilling tests for stress wave propagation and dissipation with a hammer and rods. However since many factors are involved in percussive drilling, both the Lundberg’s and Okubo’s models of a piston and rods were simplified and far from the actual products. In this study, stress waves during percussive drilling were analyzed and a new numerical model was examined for accurately simulating stress wave propagation in rods based on the one-dimensional finite element method. We adopted actual shape models for a piston and rods, instead of treating them as simple uniform cylinders in Lundberg’s and Okubo’s studies. In addition, the spring model of a sleeve-type joint proposed by Okubo et al. was employed for simulating the stress wave propagation from one rod to another. Finally, the calculated results were compared with the experimental data of percussive drilling tests. The new model can effectively simulate the stress waveform and even the oscillation in the waveform caused by the complex shape of a piston and a shank rod.
The principal objective of thispaperis to develop a controlled laboratory testfor predicting the damage of soft rocks when exposed to blast or shock waves. Shock tube experiment is proposed to generate shock/blast waves that are strong enough to create fracture in soft rocks by release of tremendous amount of energy over a small duration of time. Plaster of Parisis used as test sample in this study, as a substitute for soft rocks. The evaluation of damage is based on the energy absorbed by the sampleonexposing to the incoming shock waves. The critical energy is the measure,where the sample resistance is surpassed and the absorbed energy isreleased in the form of crack or fracture. The incident, transmitted (absorbed) and residual energy is evaluated based on the principles of gas dynamics and global mass, momentum and energy conservation equations. The rate of loading is varied by producing shock waves of different Mach number (strength). It is observed that energy absorbed by the sample varies linearly with impulseimparted.
A new scheme for measuring and simultaneously visualizing inclination of a rock structure is proposed. A mirror is installed in a mobilized zone in such a way that an observer can identify a light source in the mirror. If then the mirror rotates by a small amount due to movement of the rock structure, the observer would first find the reflected image of the light source moving in the mirror, then lose it once the rotation exceeds a limit value determined by mirror size, distance between the observer and the mirror, and the distance between the mirror and the light source. A total system of measuring inclination and visualization using this method allows a low cost monitoring system of arbitrary rock structures.
Injection-induced seismicity associated with applications, in which fluids are intensively pressed into the deep formations of the Earth's crust such as Enhanced Geothermal System (EGS), fracking shale gas, geological sequence of CO2, have attracted growing attentions. Motivated by the desire to better understand the mechanism of damaging events so that they can be avoided or mitigated, we have started an integrated study on rock fracturing and fault reactivation in multiscales. In this paper, we present some preliminary results of an ongoing experimental study utilizing acoustic emission technique in laboratory scale. We systematically carried out rock fracture tests using samples of typical sedimentary rocks collected from the Sichuan Basin, China, where a number of injection-induced seismic swarms with sizable earthquakes ranging up to M4~5 have been observed in some gas/oil reservoirs. Since most injection-induced earthquakes are located in sedimentary strata of a wide range of lithology and depth, the fracturing behaviors of such rocks are thus important. Our results indicate that the Pre-Triassic rocks in the Sichuan Basin, including dolomite or dolomitic limestone and shale are strong and demonstrate brittle fracturing behaviors. Such properties are necessary conditions for maintaining high level reservoir stress and resulting seismic fracturing.
The gas tightness of underground rock cavern tanks installed for the storage of gas and oil is preserved by sealing the rock joints around the cavern tanks with groundwater. This storage system is efficient and cost effective, because utilizing groundwater around the rock cavern tanks negates the need for expensive steel liner plates. However, the mechanisms and principles of this gas sealing method are not entirely clear.
Two conditions are important for a groundwater sealing system. The fIrst condition is that gas does not exude into the rock joints from the cavern surface. The second condition is that gas bubbles must be retained in the rock joints, even if exuding into the joints.
Ensuring higher water pressure than gas pressure satisfIes the fIrst condition. The second condition is thought to be governed by bubble buoyancy, capillary pressure, and drag force, however the effect of these factors on the second condition is not clearly understood. Therefore, we performed laboratory experiments to elucidate the mechanisms of securing gas tightness in rock joints with groundwater.
The experimental model was comprised parallel glass plates imitating an actual underground rock joint, and we observed the behavior of air bubbles injected into the gap fIlled with water. The width of the gap between the parallel glass plates and the hydraulic gradient were varied parametrically.
This paper shows the methods and results of the experiments and theoretical speculations of the reliance of gas tightness on the buoyancy, surface tension, and drag force.
Cutter forces have an indispensable place in mechanical excavation, especially selection of excavating machine. During mechanical excavation, three-dimensional forces occur on the cutter. These are cutting force or rolling force (FC and FR), normal force (FN) and sideway force (FS). Cutting force and normal force is very important variables such that torque and thrust capacity of mechanical excavator is determined via these variables in design stage. Cutter forces can be calculated or measured practically or theoretically. In laboratory rock cutting tests, cutter forces occurring on index or real cutters can be determined practically. These rock cutting tests are, full scale rock cutting test and small scale rock cutting test. Cutter forces are found theoretically by cutting theories developed by some researchers. Best way of determining cutter forces is laboratory rock cutting tests but they could be found in a few research centers around the world. So researchers are searching alternative ways of determining cutter forces.
In this study, six different rock samples are conducted to laboratory tests for determination of physical and mechanical characteristics. During this experimental period, uniaxial compressive strength, Schmidt hammer rebound value, uniaxial tensile strength, unit weight and apparent porosity parameters are determined. As well, same rock samples are conducted to small scale rock cutting test and cutting force and normal force is measured during cutting test. Data which are obtained from these tests are analyzed in SPSS program and multivariate linear regression formula are developed for cutting force (FC) and normal force (FN) both. These multivariate formula include three parameters which are uniaxial compressive strength, Schmidt hammer rebound value and apparent porosity. Other parameters (uniaxial tensile strength and unit weight) were not entered through regression. Multivariate linear formulas were tested via ANOVA test and passed this test.
Lee, J. (Department of Energy Systems Engineering, Seoul National University) | Min, K. B. (Department of Energy Systems Engineering, Seoul National University) | Rutqvist, J. (Earth Sciences Division, Lawrence Berkeley National Laboratory)
Shear slip at a fracture is a critical issue for many applications of geological engineering, such as the design of an underground repository for nuclear waste, the enhanced geothermal systems (EGS), and the geosequestration of CO2. This paper presents the new development of the TOUGH-UDEC simulator for the coupled thermal-hydraulic-mechanical analysis in fractured porous media. This paper introduces the methodology about linking TOUGH2 and UDEC for the coupled thermal-hydraulic-mechanical (THM) analysis of multiphase fluid flow, heat transfer, and deformation in fractured porous media. TOUGH2 is widely used for modeling heat transfer and multiphase, multicomponent fluid flow, and UDEC is a well-known discrete element code for rock mechanics. These two codes are executed sequentially and linked through external coupling modules which serve to pass relevant information between two codes. For the coupled THM analysis, pore pressure and temperature information calculated by TOUGH2 are updated to UDEC, and stress information calculated by UDEC is updated to TOUGH2 changed to porosity and permeability. Verification examples are presented to demonstrate the capabilities of the coupled TOUGH-UDEC simulator.
Mori, T. (KAJIMA Corporation) | Nakajima, M. (KAJIMA Corporation) | Sakaguchi, K. (Tohoku University) | Aoki, S. (Yonden Consultants Co.) | Kaji, T. (Inc.) | Nagai, K. (Shikoku Electric Power Co., Inc.) | Sasaki, K. (Shikoku Electric Power Co., Inc.)
As the initial rock stress measurement technique for design of large-scale underground cavern, the Compact Conical-ended Borehole Over-coring (CCBO) technique is increasing application performance. The stress calculation model of conventional CCBO technique is based on linear elastic model. Therefore, there is a possibility of obtaining a different value compared with the true initial stress in the anisotropic rock condition. The objective of the work is to establish an initial rock stress measurement method which can be applied to anisotropic rock mass. Especially in this study, the subject is focused on frequently used anisotropic elastic rock model, the transversely isotropic model, which has one plane of transverse isotropy and one axis of rotational symmetry orthogonal to the plane. As the first step of this study, this ordinary theory of the CCBO technique is modified so as to deal with the anisotropy problems.
Authors tried the application of the anisotropy theory in black schist which anisotropy is well developed, and compare with the results of isotropy model analysis in this paper.
Underground space is an important topic in rock mechanics and rock engineering. The utilization of the characteristics of underground space is considered, and applications to environmental preservation, the effective use of space in mega-cities with high-population densities, the protection of people's lives and property from disasters, etc., are among the topics currently being discussed.
Unfortunately, people generally have negative impressions when they think of being underground, namely, darkness, isolation, fear, etc. In order to make people feel comfortable and pleasant in underground spaces, the psychological viewpoint, in addition to the conventional engineering viewpoint, must be quite important when designing underground spaces. The authors have, therefore, investigated the relationship between mechanics and human impressions for the shapes of underground spaces.
In this paper, the psychological viewpoint is extended to a discussion on "time". Namely, the authors address the question, “How do people feel the progress of time in an underground space?” The time interval which people feel may differ from the interval in clock time. This issue is called “time estimation”. Underground spaces should be designed by taking into account the results of this time estimation. In order to investigate the time estimation for underground spaces, a psychological experiment is conducted by using video images. A questionnaire is also used to assess how people feel about the images. These video images were filmed while walking through an underground mall and along outside streets. The results of the experiments are analyzed, and the time estimation, the evaluation of human impressions and the relations among them are discussed.
As a rock engineering project, an investigation into the psychological effects of the time estimation on people will be useful for designing and utilizing underground spaces.